This invention relates to ferroelectric ceramic compositions consisting of solid solutions of Pb(Fe1/2.NB1/2)O.sub.3 - Pb(Zn1/3.Nb2/3)O.sub.3 - PbTiO.sub.3 - PbZrO.sub.3 which are contained in a prescribed ratio range.
Ferroelectric ceramics utilized to construct ceramic filters, ultrasonic wave delay line elements, piezoelectric ignition elements and piezoelectric vibrating elements are required to be polarized readily when a DC voltage is applied and to have optimum electrical characteristics, as for example, high insulating resistances (IR), high electromechanical coupling coefficients (Kp), high dielectric constants (.epsilon.), and high Poisson's ratios (.sigma.).
Heretobefore, ceramic compositions of the binary system, typically compositions of the PbTiO.sub.3 -PbZrO.sub.3 type, and ternary system, typically compositions of the Pb(Mg1/3.Nb2/3)O.sub.3 -PbTiO.sub.3 -PbZrO.sub.3 type are generally used to prepare these ferroelectric ceramics. Generally, however, these compositions are difficult to fire and their range of firing temperature is narrow. Furthermore, it is difficult to obtain fired products of homogeneous composition.
For example, compositions of the PbTiO.sub.3 -PbZrO.sub.3 type are difficult to fire because lead oxide, one of the constituents, readily vaporizes during the firing step so that the resulting ceramics lack homogeneity and denseness and their dielectric and piezoelectric properties greatly depend upon the Zr:Ti ratio.
In addition to these drawbacks, compositions of the Pb(Fe1/2.Nb1/2)-PbTiO.sub.3 -PbZrO.sub.3 type have low insulation resistance and are difficult to polarize. Furthermore, with the compositions of the Pb(Zn1/3.Nb2/3)O.sub.3 -PbTiO.sub.3 -PbZrO.sub.3 type the range of the compositions manifesting satisfactory electrical characteristics is extremely narrow. Also, in many applications of the ferroelectric ceramic compositions they are being used widely as a resonator of an energy-trapped type in electric wave filters for 1 to 10 MHz frequencies. In such applications, however, ferroelectric ceramic compositions having a Poisson's ratio of less than one-third essentially prevent the necessary energy trapping.
In the prior art it has been difficult to produce ceramic compositions having a Poisson's ratio of one-third, and it has been almost impossible to obtain ceramic compositions with a ratio of more than one-third. For example, an article in IRE, 1960 No. 2, pages 220 to 221, reports the highest Poisson's ratio for a ceramic composition as 0.328. Furthermore, the frequency variation in relation to the temperature variation (the temperature characteristics of the frequency) of an electric wave filter utilizing the prior art ceramic compositions has normally been about 100 ppm and in no instance better than 50 ppm with some superior materials.
Thus, none of these prior art compositions is suitable for obtaining ceramics with excellent electrical characteristics.